Metal detectors are used in a variety of applications for the detecting and reporting of metallic objects otherwise hidden from view by soil, clothing, packing materials, water, etc. As well as law enforcement and military applications, metal detectors find use by hobbyists, historians and others who search for artifacts or natural metallic objects in soil or underwater.
In most applications, it is very useful for the operator of a metal detector to be able to obtain information about the composition of a metal object from the response of the detector, without having to dig or search for the object in order to identify it. For instance, a hobbyist or historian searching for silver coins in the area of an old treasure trove would be uninterested in the detection of beer can pull tabs, and would prefer not to waste time digging out the large number of uninteresting and undesired, or “junk”, targets. Thus it is desirable for metal detectors to incorporate features allowing at least preliminary identification of what a metallic object might be. Such preliminary identification might include factors such as the material of composition, the size or shape of the object, and the depth or distance of the object from the detector, among others.
Since many of the kinds of metallic objects that are found are of similar size and shape (e.g. coins vs. pull tabs) and are found a similar depths in soil, the material composing the metallic object is an important factor in ascertaining whether to invest the effort to exhume the detected object.
Metal detectors can obtain information about the composition of metallic objects by means of the different responses of various metallic elements, alloys, and compounds to the electrical signals generated by metal detectors. The principle of operation of metal detectors of the present “induction” type involves the production and transmission of a time-varying, or alternating, electromagnetic wave and the detection of a responsive electromagnetic wave that is induced in metallic objects that are located near the detector by the transmitted energy. Different metallic substances, due to their differing physical properties, cause differing degrees of delay in the phase of the induced time-varying electromagnetic wave as detected by the receiving apparatus of the metal detector. While the material composing a metallic object does not unambiguously define what the object is, it does allow for objects unlikely to be of interest to be rejected and others to be identified as worthy of attempts at recovery.
Various metallic elements (copper, etc.), alloys (stainless steel, etc.) and compounds (ferrous salts, etc.) delay the phase of an induced oscillating electromagnetic field to differing degrees; for instance nickel coins and aluminum foil delay the phase less than do copper pennies and pull tabs, which delay less than do dimes and quarters. Thus, some items of usual interest such as dimes and quarters are relatively easy to distinguish from aluminum foil, and it is known to design a metal detector with a settable threshold function that would report dimes and quarters but not report aluminum foil, based on the widely separated positions of these objects on the phase delay scale. However, other objects of potential interest such as nickels lie on the phase delay scale between objects that would likely be regarded as junk targets such as aluminum foil and beverage can pull tabs.
The prior art has not squarely addressed this problem. For example, U.S. Pat. No. 4,700,139 permits a user to define a single excluding region of phase delay within which a target will not be reported. U.S. Pat. No. 5,148,151 requires the user to pre-program the detector regarding accept or reject criteria for all regions of the phase delay scale in which the user desires to make a specification.
U.S. Pat. No. 4,677,384 provides a discriminate circuit that can be overridden by a target select circuit that allows reporting of target information that would otherwise be suppressed as not falling within an acceptable range of phase delay. However, it is not recognized that it is often desirable to suppress the reporting of target information that otherwise would be reported as falling within the acceptable range of phase delay.
It is a further problem not addressed by the prior art that the user involvement in defining accept and reject criteria has risen in complexity in proportion to the amount of definition required. Accordingly, there is a need for a metal detector having a plurality of discrimination regions with corresponding exception spaces therein that provides for easily defining relatively complex discrimination criteria.